It is known to employ composite tools comprising a polycrystalline diamond active part for effecting machining operations involving mechanical removal of a hard material.
Such tools are employed for drilling rocks in the mining or oil exploitation field, for cutting coal or other natural materials which are extracted, or for machining metals.
Composite tools are known which comprise a support of cemented metal carbide, for example tungsten carbide, and a polycrystalline diamond active part having an inner surface ensuring the connection with the support and an outwardly facing working surface for contacting the material to be machined.
The connection between the polycrystalline diamond active part and the tool support is of the metallurgical connection type usually employing a metal such as cobalt which may also be employed for the connection between the particles of diamond of the active part.
These composite tools are obtained by compacting and sintering methods employing high temperatures and very high pressures.
Composite tools having a diamond-impregnated working surface are advantageously employed in machining operations carried out on rocks, such as drilling, cutting or excavation.
In order to ensure that these operations are carried out in a satisfactory manner, as in any other machining operation involving the removal of material, it is necessary to achieve efficient cooling of the zone of contact between the tool and the material being machined.
There has, for example, been proposed in FR-A-2,089,415 a tool whose active part is constituted by diamond crystals directly interconnected with cobalt, nickel or iron and having a very small volume relative to the volume of the carbide support.
In the case of the drilling of rocks with tools constituted by sintered polycrystalline diamond picks bound by the cobalt, cooling is ensured by the circulation of a fluid which sweeps across the zone of contact between the tool and the rock, i.e., the zone of contact of the working surface of the polycrystalline diamond tool.
Notwithstanding this cooling, the stresses applied to the tool, depending on the type of rock encountered during drilling, may be such that the heating of the active part of the tool becomes excessive and results in thermal degradation of this part of the tool by intergranular cracking or decohesion of the zone of junction between the active part and the support. Consequently, the life of the tools or tool elements having a composite structure is reduced.
It has been proposed in FR-A-2,380,845 provide a network of intercommunicating pores throughout the volume of the tool element constructed in a composite manner, the total volume of the pores being as much as 5 to 30% of the volume of the tool element.
However, such a tool, while its resistance to thermal degradation is improved, has mechanical characteristics which are distinctly inferior to those of composite tools constructed in a dense form and including an active part constituted by particles of polycrystalline diamond bound together with a metal such as cobalt.
The working surfaces of conventional composite tools usually have a rounded shape, the polycrystalline diamond active part usually being hemispherical. This renders the tool very tough, but, when the tool has undergone a certain amount of wear, the cutting efforts have a tendency to increase and this results in increased heating and therefore increased thermal degradation of the active part of the tool.
Furthermore, prior art tools constructed in the composite form usually do not permit easy and rapid break-up and elimination of the cuttings formed by removal of material in the course of the machining. There is consequently an increase in the cutting effort and heating of the tools.